PROJECT SUMMARY/ABSTRACT Prostate cancer (PCa) is the most frequently diagnosed cancer among American men. PCa has the largest racial disparities of any cancer in which African Americans (AAs) have the highest PCa burden. Although health behaviors such as physical activity, low sedentary behavior, and healthy eating can reduce morbidity and mortality among PCa survivors, AA cancer survivors do not meet recommended levels of health behaviors and report poorer health behaviors than others. As romantic partners are often reported as a primary source of support for AA PCa survivors, integrating partners into AA PCa survivorship care and broadening the focus of behavioral interventions from the individual (survival) to the survivor-partner dyad may make health behaviors more easily adopted and potentially maintained. Despite the importance and novelty of targeting the survivor- partner dyad as a single unit of care, to date, no evidence exists that dyadic lifestyle interventions are superior to individual-only interventions. We argue that this result is partly due to the fact that these dyadic lifestyle interventions were simply adding partners in the existing individual-only interventions and did not take on a truly dyadic perspective. Before implementing another dyadic lifestyle intervention, micro-level investigations of interactions between survivors and partners is necessary to pinpoint how AA PCa survivors and partners facilitate or hinder each other's HBs in their natural, everyday lives. Dyadic coping indicates that couples respond to stressors as interpersonal units. It includes supportive/unsupportive (e.g., providing practical help, showing disinterest), delegate (e.g., taking on things that the partner normally does), negative (e.g., mutual avoidance), and common dyadic coping (e.g., joint problem solving). Thus, dyadic coping goes beyond the exchange of social support. To date, virtually no studies have tested the impacts of dyadic coping on HBs among AA survivors and partners. The objective of the present study is to fill these gaps using ecological momentary assessment to eventually develop more effective lifestyle interventions for AA PCa survivors and partners. A total of 120 dyads (i.e., 240 individuals) who are AA adult survivors diagnosed with non-metastatic PCa and their romantic partners will be asked to complete four assessments per day for 14 consecutive days on a smartphone. During each assessment, participants will be asked to complete a brief survey regarding their health behaviors, contexts of health behaviors, stress, and dyadic coping. Physical activity and sedentary behavior will be assessed via accelerometer. At completion of this study we expect to achieve the following milestones: 1) candidate dyadic coping as a determinant of behavioral change of both survivors and partners, 2) identify critical time points of intervention delivery, and 3) determine intervention characteristics. The proposed study along with our on-going pilot study of AA PCa survivors and their partners will inform the rigorous development of a theory-based dyadic lifestyle intervention in a vulnerable survivorship population with the ultimate goal to improve overall survival and reduce cancer incidence.